HEWLETT PACKARD (HP) 2S
Service Manual

The basic voltage range of the servo systems i s 0 to 5 millivolts for the Y axis and 0 to 7. 5 millivolts f o r the X axis for full scale travel. Operation with g r e a t e r voltages is obtained by means of a step attenuator inserted between the input terminals and the balance circuit. Each range step may be made continuously variable by operation of a transfer switch which i n s e r t s a variable potentiometer and extends that range to about five times i t s fixed value. The range position i s selected so that the amplitude of the input data will be within the travel limits of the instrument. Included in the input c i r cuit is a variable filter which minimizes undesirable noise and ripple disturbances in the input signals. After passing through the input filter and attenuator, the input signal is applied to the balance circuit, where it i s cancelled by a n internally supplied opposing voltage derived f r o m a reference cell and potentiometer. Under these conditions there i s no signal output f r o m the balance circuit and the servo system is a t rest. When the input signal changes to a new value it is no longer canceled in the balance circuit and the unbalance voltage, o r e r r o r signal, is applied to the chopper where it is converted to a 60-cycle form. The A C output of the chopper is amplified and applied to the control winding of a two-phase servo motor. The motor is mechanically coupled to the balance circuit potentiometer and thus changes the balance voltage until it again cancels the new value of input signal. If the input data is constantly changing at r a t e s within the capabilities of the instrument, this rebalancing action is continuous. Thus the positions of the balance circuit potentiometers and of the pen and c a r r i a g e , to which they a r e coupled, a r e always directly proportional to the amplitudes of the signals a t the respective input terminals. The use of the X-axis with a variable input voltage o r as a sweep circuit is selectable by a function switch on the r e a r panel. Operation of the time base i s dependent on the f a c t that the charging current taken by a condenser will a s s u m e a constant value when the charging voltage i s controlled so a s to inc r e a s e at a uniform rate. Existing facilities of the r e c o r d e r a r e utilized to apply a linearly increasing voltage to the X-axis servo amplifier causing the pen to advance at a uniform r a t e in total sweeps dependent on the constants switched into the circuit. These constants a r e so selected during manufact u r e that the time required for full scale pen t r a v e l equal, in seconds, the numerical values of the millivolt steps of the X-axis range selector. The Model 2s Autograf consists of t h r e e major assemblies: novable aluminum case. >ase plate assembly, mounting the amplifier chassis (which contains m e chopper, two high gain amplifier channels and a power supply for the amplifier), the various input plugs (mounted on a side panel), the r e f e r ence cells, the p r i m a r y power control panel, and the vacuum pump.

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